A technology for producing lenses by press-forming a glass material without requiring performing a glass-polishing step has eliminated complicated steps that were necessary in conventional methods of producing an optical element forming mold, thereby having enabled easier production of lenses at lower cost. Properties required for a mold material used for such press forming of a glass optical element include excellence in hardness, heat resistance, releasability, mirror finishing property, and the like.
Conventionally, there have been proposed, as a mold material of this kind, many materials such as metal, ceramics, and materials on each of which metal or ceramics are coated. In particular, a mold produced by forming a carbon film such as a diamond-like carbon film, a hydrogenated amorphous carbon film (a-C:H film), a hard carbon film, or a tetrahedral amorphous carbon film (ta-C film) is good in releasability between the mold and glass, and hence the mold has the advantage that fusion-bonding between the carbon film and glass is unlikely to occur.
However, the mold had, in general, low adhesiveness with the carbon film, and after glass press forming operations were repeated more than several hundred times, the carbon film was partially peeled, and hence sufficient formability was not given to the mold occasionally. That is, the mold had a problem with durability, leading to the high cost of a press-formed product. Further, when a desire for press forming of high refractive index glass has been increasing with progress in downsizing of digital cameras and the like, the press forming of high refractive index glass performed at as high a forming temperature as 650° C. or more has involved a big problem in that the mold produced by using any of the diamond-like carbon film, the a-C:H film, and the hard carbon film have poor durability.
Then, there is known, as a carbon film having good heat resistance, a ta-C film obtained by using a filtered cathodic vacuum arc process (FCVA process) as disclosed in Patent Literature 1. A conventional diamond-like carbon film (hard carbon film) produced by using a methane-based gas contains hydrogen atoms, and hence, when the film is formed under high temperature, bonds between a carbon atom and a hydrogen atom are likely to be cut, resulting in graphite bonding (sp2 bonding) between carbon atoms and leading to reduction in the hardness of the film. On the other hand, as the ta-C film is produced by the FCVA process using graphite as a material, it is possible to obtain a hydrogen-free, diamond-like carbon film (sp3 bonding having high strength).
However, according to the above-mentioned method of forming a ta-C film by the FCVA process, while fine particles of a cathode material occurring simultaneously with ion release from a cathode spot are being trapped and removed during plasma-magnetic transportation, only carbon ions are allowed to reach a mold matrix (forming mold substrate), thereby forming a film. The FCVA process described above had the problem that the gradient peripheral portions of the resultant mold had low heat resistance. In particular, the gradient peripheral portions of a mold having a shape with a large angle between principal traces (angle made between the optical axis center of a mold and the normal direction at an optically effective diameter position) were apt to have inferior heat resistance, compared with the top portion of the mold. Thus, the ta-C film became liable to be peeled at the peripheral portions of the mold as the number of press forming increased, resulting in degradation of durability.